Can you upgrade aircrafts at fractionals? Like Flexjet for example, if you were a Lear FO can you move up to the Challenger or are you required to stick with the lear?

I don't know about Flex- but at NJ you can upgrade or transition as captain into whatever airplane your seniority will hold after a two year seat lock or if something that pays better opens you can go to that in less than 2 years. It has been common lately for Falcon FOs to go to Excel CA since the pay is so much higher.